Preparation of (NH4)2MoOBr5 and [pyrHMoOBr4]n

The following preparations are unpublished documents from a Masters Thesis
completed at South Dakota State University in May 1978. The structures of the
compounds and how they are related to Mo(V) solutions are discussed on another
page on this site. See the link at the bottom of this page. References to
two Journals are listed at the bottom of this page.

Concentrated Molybdenum(V) Solutions in Hydrobromic Acid

Most of the oxobromomolybdate(V) compounds prepared in this study
were precipitated from concentrated molybdenum(V) solutions in
hydromic acid. The procedure for it and preparing molybdenum (V)
solutions was a modification of one given by Saha and Bannerjee.

Approximately 40 ml of concentrated hydrobromic acid and was added
to 5 g of molybdenum trioxide in the 250 ml beaker. A watch glass
was put over the beaker and the mixture was refluxed gently on a
hot plate in a chemical hood. After about two hours, the watch
glass was removed and the thick dark brown solution was allowed to
evaporate to a desired volume. The solution volume selected depended
on the soluability of the compound to be precipitated.
If necessary, the volume was adjusted by adding concentrated
hydrobromic acid. The solution was allowed to cool and was
suction filtered using a fine porosity sintered class crucible and
a water aspirator. It was used immediately after filtration.

Ammonium Oxopentabromomolybdate(V) [ (NH4)2MoOBr5 ]

Molybdenum trioxide (10 gm, 69.4 mmole) was reacted with hydrobromic
acid as described above to produce a molybdenum (V) solution with
a volume of about 15 ml in a 150 ml beaker. Ammonium bromide (9.50
gm, 97.0 mmole)
was added to the molybdenum (V) solution. Hydrogen bromide was
bubbled through the solution. After it had been saturated
with the gas, the beaker was cooled to about -20 degrees centigrade
by holding it in the mouth of a Dewar flask containing liquid
nitrogen.

The Brown, crystalline precipitate was isolated from the cold
solution, using suction filtration in a closed system. Air
sucked into the filter was first dried by passing it through
anhydrous calcium chloride first and then potassium hydroxide.
Filtration without a dry air flow through the filter cause the
surface to become discolored, indicating possible decomposition.
The product was washed twice with 5 ml of a cold and saturated
ethanol-hydrogen bromide solution, and dried in a vacuum dessicator
under reduced pressure over solid potassium hydroxide. A water
aspirator was used to produce a partial vacuum in the dessicator.

Product weight was 22.21 g. ( % yield = 83.7% ).

A portion of the above product (6.00 gm, 10.96 mmole) was dissolved
in 2 M hydrobromic acid in a 50 ml beaker. The solution was
saturated with hydrogen bromide and cooled above liquid nitrogen.
The product was separated, washed and dried as described above.

Pyridinium Catena-u-oxo-tretrabromomolybdate(V) [ (pyrhMoOBr4)n ]

Molybdenum trioxide 4.00 g, (27.8 mole) was reacted with
concentrated hydrobromic acid to produce a 30 ml
molybdenum(V) solution in a 150 ml Baker. Pyridinium hydrobromide
was prepared by dissolving 40 ml of pyridine in 5 ml of ethanol,
saturating the solution with hydrogen bromide, and heating it to
evaporate the ethanol and excess hydrogen bromide. The resulting
pyridinium hydrobromide (3.50 gm, 21.9 mmole) was dissolved
in a minimum volume of 7 M hydrobromic acid. The solution was
poured with stirring into the molybdenum(V) solution, resulting
in a heavy brown precipitate.

The mixture was saturated with hydrogen bromide and the sealed
beaker was cooled in an ice bath. The resulting red crystalline
solid was isolated, washed and dried as described for the ammonium
compound above.

Product weight was 9.34 g ( % yield = 78.1% ).

A portion of the product (6.92 gm, 12.7 mmole) was recrystallized
by dissolving in 7M hydrobromic acid and saturating with hydrogen
bromide. The product was isolated, washed and dried as described
above.

A different crystallization procedure was used to produce crystals
of the compound suitable for an Xray crystallographic analysis.
A sample of the compound (1.00 g, 1.83 mmole) was dissolved in
a minimum volume of boiling 8.6 M hydrobromic acid. The solution
was filtered, transferred to a thirty gm weighing bottle, and
saturated with hydrogen bromide. The bottle was tightly capped
and allowed to cool slowly to room temperature. While the solution
was still hot, a few crystals of the compound were added to
prevent super saturation, and the solution was allowed to stand
for 48 hours. About 0.10 g of reddish brown crystalline product
was separated, washed and dried as described above.